In a modern steel works producing a wide range of steel products of differing steel grades, it is important to be able to carry out non-destructive testing of steel specimens to establish their correct grades. One current method, known as the "Forster" method, relies on a comparison between the magnetic properties of a steel specimen to be tested and a "standard" sample which has been chemically analysed. The test specimen and the standard sample are inserted into separate coils, each having primary and secondary windings. The primary windings of the two coils are subjected to a common energizing current of fixed frequency and a comparison is made between output signals induced in the secondary windings of the two coils. The output signals may be applied to a comparison circuit which generates a display on a screen.
The Forster method of steel classification has several limitations and problems in practice. These include:
1. A highly trained operator is required to interpret the trace or screen display to enable satisfactory information to be gained.
2. Since the operator must visually access a visual display, estimating the amplitude and shape of a wave form generated by the test sample, only a qualitative idea is obtined of possible differences between successive samples being tested.
3. Since the test method is not quantitative and since there are always some variations in the physical and chemical properties even within a given grade of steel, it is very difficult to be certain whether an observed changed in the display constitutes a real change in grade or only a variation explainable by within grade variations.
4. The method is very sensitive in variations in temperature between the test specimen and the standard sample.
5. Results depend on chemical analysis of the initial standard sample. Once it is considered that a change in grade has occurred the new grade cannot be determined without further recourse to sampling and subsequent chemical analysis.
The present invention has arisen from work aimed at developing an improved magnetic testing method which will enable a more accurate and convenient testing procedure.